When calling Filesystem::mirror() with a $iterator parameter (to customize the list of files/folders to mirror), and when using the delete option on top of that, things break.

This new test covers the problem (but do not fix it). I have been scratching my head at it for an hour but I'm really having trouble with the implementation of Filesystem::mirror().

I believe the bug was introduced following #23473, more specifically:

- $origin = str_replace($targetDir, $originDir, $file->getPathname());
+ $origin = $originDir.substr($file->getPathname(), $targetDirLen);

Here is my debug session right before the exception:

Here are the values of the variables:

$originDir = '/tmp/source';
// I used a weird name on purpose so that it's longer than `source` to trigger the bug
$targetDir = '/tmp/targettarget';

// Here is the computed value of the file we are trying to delete:
$origin = '/tmp/sourcece1';
// it should be `/tmp/source/source1` instead if I understand correctly

I'm out of ideas for tonight sorry! I don't know if PR with failing tests are useful at all? I won't mind if you close it.